The present disclosure relates generally to golf balls and their manufacture. In particular, the present disclosure relates to a golf ball having a thermoplastic polyurethane cover, where the thermoplastic polyurethane cover is crosslinked by the presence of a polyurethane coating layer.
The game of golf is an increasingly popular sport at both the amateur and professional levels. A wide range of golf balls are known, which have any of a variety of constructions made from a diversity of different types of materials. Golf ball covers, in particular, are generally divided into two types: thermoplastic covers and thermoset covers.
Thermoplastic polymer materials may be reversibly melted, and so may be used in a variety of manufacturing techniques such as compression molding or injection molding that take advantage of this property. On the other hand, thermoset polymer materials are generally formed by mixing two or more components to form a cured polymer material that cannot be re-melted or re-worked. Each type of polymer material presents advantages and disadvantages when used to manufacture golf ball covers.
Thermoplastic materials used for golf ball covers usually include ionomer resins, highly neutralized acid polymer compositions, polyamide resins, polyester resins, polyurethane resins, and mixtures thereof. Among these, ionomer resin and thermoplastic polyurethane resin are popular materials for golf ball covers. One popular thermoplastic ionomer material is available from E. I. DuPont de Nemours and Company under the trademark name Surlyn®
However, ionomer resin covered golf balls suffer from the problem that the cover surface may be scraped off by grooves on a clubface during repeated shots, particularly with irons. In other words, ionomer covers have poor scuff resistance. Also, ionomer covered balls usually have inferior spin and feel properties as compared to balata rubber or thermoplastic polyurethane covered balls. The use of softer ionomer resins for the cover will improve spin and feel to some extent, but will also compromise the golf ball's resilience because such balls usually have a lower coefficient of restitution (C.O.R.). Furthermore, the scuff resistance of such softer ionomer covers is often still not satisfactory.
Thermoplastic polyurethane elastomers may also be used as the cover material, as described in (for example) U.S. Pat. Nos. 3,395,109, 4,248,432 and 4,442,282, the disclosures of which are herein incorporated by reference in their entireties. However, the thermoplastic polyurethane elastomers disclosed therein do not satisfy all the requirements of moldability, hitting feel, control, resilience, and scuff resistance.
On the other hand, thermoset polymer materials such as polyurethane elastomers, polyamide elastomers, polyurea elastomers, diene-containing polymer, crosslinked metallocene catalyzed polyolefin, and silicone, may also be used to manufacture golf ball covers. Among these materials, thermoset polyurethane elastomers are popular.
Many attempts, such as are described in U.S. Pat. Nos. 3,989,568, 4,123,061, 5,334,673, and 5,885,172, have been made to use thermoset polyurethane elastomers as a substitute for balata rubber and ionomer resins. Thermosetting polyurethane elastomers are relatively inexpensive and offer good hitting feel and good scuff resistance. In particular, thermoset polyurethane elastomers may present improvements in the scuff resistance as compared to softened ionomer resin blends. However, thermoset materials require complex manufacturing processes to introduce the raw material and then effect a curing reaction, which causes the manufacturing process to be less efficient.
Therefore, various attempts have been made in the golf ball arts to combine the processing advantages of thermoplastic materials with the improved physical properties of thermoset materials. In the particular context of thermoplastic polyurethane covers, a variety of technologies have been used to improve the physical properties of thermoplastic polyurethane covers by exposing the cover to increased amounts of isocynate.
U.S. Pat. No. 7,217,200 to Matroni et al. discloses one such approach, the disclosure of which is herein incorporated by reference in its entirety. This patent discloses a process commonly referred to as isocynate dipping, whereby a thermoplastic polyurethane golf ball cover is dipped into a isocynate curing agent bath. The isocyanate bath crosslinks the thermoplastic polyurethane cover material, thereby increasing its durability and scuff resistance.
U.S. Patent Application No. 2007/0142127 to Tarao discloses another such approach, the disclosure of which is herein incorporated by reference in its entirety. This patent application discloses a golf ball with a thermoplastic polyurethane cover, wherein the cover also includes microcapsules containing a polyisocyanate. After the cover is molded, the golf ball is subsequently heated so as to break the microcapsules and release the polyisocyanate into contact with the thermoplastic polyurethane cover material. The cover thereby achieves increased abrasion resistance and durability.
Finally, U.S. Pat. No. 7,481,722 to Higuchi discloses another method of exposing a thermoplastic polyurethane golf ball cover to increased amounts of isocynate. The disclosure of U.S. Pat. No. 7,481,722 to Higuchi is herein incorporated by reference in its entirety. Specifically, this patent discloses a process for forming a golf ball cover whereby the cover is made from a mixture of a thermoplastic polyurethane, and a polyisocyanate mixed with a thermoplastic material that is not reactive with the polyisocyanate. After this mixture is molded into a golf ball cover, the resulting golf ball is then heated in order to cause the polyisocyanate to crosslink the thermoplastic polyurethane cover material.
However, known technologies for increasing the durability of thermoplastic polyurethane such as the above mentioned examples suffer from various drawbacks and disadvantages. For example, such approaches commonly require extra processing steps in the manufacture of the golf ball, or require that the cover layer be formed from an unusual particular type of thermoplastic polyurethane.
Therefore, there exists a need in the art for a golf ball and related methods of manufacture with thermoplastic polyurethane cover layers having improved physical properties that can be manufactured in an efficient manner.